Physics and Engineering of Radiation Detection

4.1. Properties of Liquids 225

X+

Radiation

Incident

Hole

+ +

Charge Pair Creation Charge Transfer

+

X

M

M

Electron

Figure 4.1.4: Creation of an electron hole pair in a moleculeM

with a subsequent transfer of positive charge to another molecule

X.

Recombination ofX+:The ionX+can also recombine with an electron to

form a neutral molecule (see Fig.4.1.5). A photon may be emitted during the

process as well.

• M

X+

+ +

+

Hole

+ +

X

Electron

Charge Transfer

X

Photon

Emitted

Recombination

Figure 4.1.5: Recombination ofX+with a free electron. During

the process a photon may or may not be emitted depending on

thetypeofmoleculeX. A multiatomic molecule having lots of

vibrational and rotational energy states can also dissociate with

a radiationless transition.

The recombination rate of electron hole pairs in a liquid is generally higher than
electron ion recombination rate in gases due mainly to the spatial proximity of
molecules. Also, as we saw earlier, the number of charge pairs created in liquids
is lower than in gases for the same amount of deposited energy. Hence one should
expect that the number of charges available in a liquid to create a measurable out-
put pulse is lower than in gases. This makes the use of liquefied gases to detect
and measure very low level fluxes somewhat impractical. Therefore apart from the
liquid scintillators, the liquid filled detectors are generally used in moderate to high
radiation fields.